U.S. patent application number 14/114587 was filed with the patent office on 2015-11-12 for exhaust purification system of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is Yuki HABA, Kohei YOSHIDA. Invention is credited to Yuki HABA, Kohei YOSHIDA.
Application Number | 20150322834 14/114587 |
Document ID | / |
Family ID | 51299343 |
Filed Date | 2015-11-12 |
United States Patent
Application |
20150322834 |
Kind Code |
A1 |
HABA; Yuki ; et al. |
November 12, 2015 |
EXHAUST PURIFICATION SYSTEM OF INTERNAL COMBUSTION ENGINE
Abstract
A hydrocarbon feed valve and an exhaust purification catalyst
are arranged in an engine exhaust passage. A first NO.sub.X
purification method which injects hydrocarbons from the hydrocarbon
feed valve by a predetermined period to thereby remove NO.sub.X
which is contained in the exhaust gas and a second NO.sub.X
purification method which makes the air-fuel ratio of the exhaust
gas which flows into the exhaust purification catalyst rich to make
the exhaust purification catalyst release the stored NO.sub.X when
the NO.sub.X which is stored in the exhaust purification catalyst
exceeds a first allowable value are selectively used. Hydrocarbons
are injected from the hydrocarbon feed valve by the predetermined
period, and when the NO.sub.X which is stored in the exhaust
purification catalyst exceeds a second allowable value which is
smaller than the first allowable value, the air-fuel ratio of the
exhaust gas which flows into the exhaust purification catalyst is
made rich.
Inventors: |
HABA; Yuki; (Mishima-shi,
JP) ; YOSHIDA; Kohei; (Gotenba-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HABA; Yuki
YOSHIDA; Kohei |
Mishima-shi
Gotenba-shi |
|
JP
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi
JP
|
Family ID: |
51299343 |
Appl. No.: |
14/114587 |
Filed: |
February 5, 2013 |
PCT Filed: |
February 5, 2013 |
PCT NO: |
PCT/JP2013/052608 |
371 Date: |
October 29, 2013 |
Current U.S.
Class: |
60/285 |
Current CPC
Class: |
B01D 2251/208 20130101;
F01N 2900/1602 20130101; F02D 41/1446 20130101; F01N 3/2033
20130101; B01D 2258/012 20130101; B01D 53/9495 20130101; F01N
2900/1614 20130101; B01D 2257/404 20130101; F02D 41/1475 20130101;
Y02T 10/26 20130101; B01D 53/9431 20130101; F01N 3/0871 20130101;
F02D 41/0275 20130101; Y02T 10/12 20130101; F01N 3/0814 20130101;
F01N 3/0842 20130101 |
International
Class: |
F01N 3/08 20060101
F01N003/08; F02D 41/02 20060101 F02D041/02; B01D 53/94 20060101
B01D053/94 |
Claims
1. An exhaust purification system of an internal combustion engine
in which an exhaust purification catalyst is arranged in an engine
exhaust passage and a hydrocarbon feed valve is arranged in the
engine exhaust passage upstream of the exhaust purification
catalyst, a precious metal catalyst is carried on an exhaust gas
flow surface of the exhaust purification catalyst and a basic
exhaust gas flow surface part is formed around the precious metal
catalyst, the exhaust purification catalyst has a property of
reducing NO.sub.X which is contained in exhaust gas if making a
concentration of hydrocarbons which flow into the exhaust
purification catalyst vibrate by within a predetermined range of
amplitude and by within a predetermined range of period and has a
property of being increased in storage amount of NO.sub.X which is
contained in the exhaust gas if making the vibration period of the
hydrocarbon concentration longer than the predetermined range, a
first NO.sub.X removal method which injects hydrocarbons from the
hydrocarbon feed valve by a predetermined injection period to
thereby remove the NO.sub.X which is contained in the exhaust gas
and a second NO.sub.X removal method which makes an air-fuel ratio
of the exhaust gas which flows into the exhaust purification
catalyst rich to make the exhaust purification catalyst release a
stored NO.sub.X when the NO.sub.X which is stored in the exhaust
purification catalyst exceeds a first allowable value are
selectively used, and in the second NO.sub.X removal method, the
period by which the air-fuel ratio of the exhaust gas which flows
into the exhaust purification catalyst is made rich is longer than
said predetermined injection period, wherein, temperature regions
which the exhaust purification catalyst can take at the time of
engine operation are divided into three regions of a low
temperature region, an intermediate temperature region, and a high
temperature region, in the high temperature region, an NO.sub.X
removal action by the first NO.sub.X removal method is performed,
in the low temperature region, an NO.sub.X removal action by the
second NO.sub.X removal method is performed, and in the
intermediate temperature region, hydrocarbons are injected from the
hydrocarbon feed valve by said predetermined injection period and,
when the NO.sub.X which is stored in the exhaust purification
catalyst exceeds a predetermined second allowable value of a value
smaller than the first allowable value, the air-fuel ratio of the
exhaust gas which flows into the exhaust purification catalyst is
made rich.
2. The exhaust purification system of an internal combustion engine
as claimed in claim 1 wherein when the second NO.sub.X removal
method is being performed and the air-fuel ratio of the exhaust gas
which flows into the exhaust purification catalyst should be made
rich, additional fuel is fed into a combustion chamber so that the
air-fuel ratio of the exhaust gas which is exhausted from the
combustion chamber is made rich.
3. The exhaust purification system of an internal combustion engine
as claimed in claim 1 wherein said intermediate temperature region
is a range where, when the NO.sub.X removal action by the second
NO.sub.X removal method is being performed, the NO.sub.X removal
rate continues to fall when a temperature of the exhaust
purification catalyst rises.
4. The exhaust purification system of an internal combustion engine
as claimed in claim 1 wherein said second allowable value is made
smaller the higher the temperature of the exhaust purification
catalyst.
5. The exhaust purification system of an internal combustion engine
as claimed in claim 1 wherein in said intermediate temperature
region, injection of hydrocarbons from the hydrocarbon feed valve
is suspended while the air-fuel ratio of the exhaust gas which
flows into the exhaust purification catalyst is made rich.
6. The exhaust purification system of an internal combustion engine
as claimed in claim 1 wherein in said intermediate temperature
region, when the NO.sub.X removal action by the first NO.sub.X
removal method is being performed, an amount of NO.sub.X which is
stored in the exhaust purification catalyst is calculated from an
amount of NO.sub.X which is exhausted from the engine, an amount of
reduction of NO.sub.X which is determined from an operating state
of the engine, and a NO.sub.X storage rate which is determined from
temperature of the exhaust purification catalyst and, when the
calculated amount of NO.sub.X exceeds said second allowable value,
the air-fuel ratio of the exhaust gas which flows into the exhaust
purification catalyst is made rich.
7. The exhaust purification system of an internal combustion engine
as claimed in claim 6 wherein a temperature difference arises in
the exhaust purification catalyst and, when there is a temperature
region which is higher than a detected temperature of the exhaust
purification catalyst in the exhaust purification catalyst, said
amount of reduction of NO.sub.X is increased.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purification
system of an internal combustion engine.
BACKGROUND ART
[0002] Known in the art is an internal combustion engine in which
an exhaust purification catalyst is arranged in an engine exhaust
passage and a hydrocarbon feed valve is arranged upstream of the
exhaust purification catalyst in the engine exhaust passage, the
exhaust purification catalyst has the property of reducing the
NO.sub.X which is contained in exhaust gas if making a
concentration of hydrocarbons which flow into the exhaust
purification catalyst vibrate by within a predetermined range of
amplitude and by within a predetermined range of period and has the
property of being increased in storage amount of NO.sub.X which is
contained in the exhaust gas if making the vibration period of the
hydrocarbon concentration longer than the predetermined range, a
first NO.sub.X removal method which injects hydrocarbons from the
hydrocarbon feed valve by a predetermined injection period to
thereby remove the NO.sub.X which is contained in the exhaust gas
and a second NO.sub.X removal method which makes the air-fuel ratio
of the exhaust gas which flows into the exhaust purification
catalyst rich to make the exhaust purification catalyst release the
stored NO.sub.X when the NO.sub.X which is stored in the exhaust
purification catalyst exceeds the allowable value are selectively
used, and in the second NO.sub.X removal method, the period by
which the air-fuel ratio of the exhaust gas which flows into the
exhaust purification catalyst is made rich is longer than the
above-mentioned predetermined injection period (for example, see
Patent Literature 1).
[0003] In this internal combustion engine, when the temperature of
the exhaust purification catalyst is high, the NO.sub.X removal
action by the first NO.sub.X removal method is performed, while
when the temperature of the exhaust purification catalyst is low,
the NO.sub.X removal action by the second NO.sub.X removal method
is performed. In this regard, in this internal combustion engine,
when the NO.sub.X removal action by the first NO.sub.X removal
method is being performed, the exhaust purification catalyst stores
NO.sub.X. If the amount of NO.sub.X which is stored in this exhaust
purification catalyst increases, the NO.sub.X removal rate at the
time when the NO.sub.X removal action by the first NO.sub.X removal
method is being performed ends up falling. Therefore, in this
internal combustion engine, when the NO.sub.X removal action by the
first NO.sub.X removal method is being performed, when the NO.sub.X
which is stored in the exhaust purification catalyst increases, the
amount of injection of hydrocarbons from the hydrocarbon feed valve
is increased to make the air-fuel ratio of the exhaust gas which
flows into the exhaust purification catalyst rich and thereby make
the exhaust purification catalyst release the stored NO.sub.X.
CITATIONS LIST
Patent Literature
[0004] Patent Literature 1: WO2011/053117A1
SUMMARY OF INVENTION
Technical Problem
[0005] However, in this internal combustion engine, in the case
where the NO.sub.X removal action by the first NO.sub.X removal
method is being performed, only restoration of the NO.sub.X removal
rate when the NO.sub.X removal rate falls is considered. Further
improvement of the NO.sub.X purification rate when the NO.sub.X
removal action by the first NO.sub.X removal method is being
performed is not considered at all. An object of the present
invention is to provide an exhaust purification system of an
internal combustion engine which is designed so that a higher
NO.sub.X purification rate is obtained compared to when the
NO.sub.X removal action by the first NO.sub.X removal method is
being performed and when the NO.sub.X removal action by the second
NO.sub.X removal method is being performed.
Solution to Problem
[0006] According to the present invention, there is provided an
exhaust purification system of an internal combustion engine in
which an exhaust purification catalyst is arranged in an engine
exhaust passage and a hydrocarbon feed valve is arranged in the
engine exhaust passage upstream of the exhaust purification
catalyst, a precious metal catalyst is carried on an exhaust gas
flow surface of the exhaust purification catalyst and a basic
exhaust gas flow surface part is formed around the precious metal
catalyst, the exhaust purification catalyst has a property of
reducing NO.sub.X which is contained in exhaust gas if making a
concentration of hydrocarbons which flow into the exhaust
purification catalyst vibrate by within a predetermined range of
amplitude and by within a predetermined range of period and has a
property of being increased in storage amount of NO.sub.X which is
contained in the exhaust gas if making the vibration period of the
hydrocarbon concentration longer than the predetermined range, a
first NO.sub.X removal method which injects hydrocarbons from the
hydrocarbon feed valve by a predetermined injection period to
thereby remove the NO.sub.X which is contained in the exhaust gas
and a second NO.sub.X removal method which makes an air-fuel ratio
of the exhaust gas which flows into the exhaust purification
catalyst rich to make the exhaust purification catalyst release a
stored NO.sub.X when the NO.sub.X which is stored in the exhaust
purification catalyst exceeds a first allowable value are
selectively used, and in the second NO.sub.X removal method, the
period by which the air-fuel ratio of the exhaust gas which flows
into the exhaust purification catalyst is made rich is longer than
the above-mentioned predetermined injection period, wherein
temperature regions which the exhaust purification catalyst can
take at the time of engine operation are divided into three regions
of a low temperature region, an intermediate temperature region,
and a high temperature region, in the high temperature region, an
NO.sub.X removal action by the first NO.sub.X removal method is
performed, in the low temperature region, an NO.sub.X removal
action by the second NO.sub.X removal method is performed, and in
the intermediate temperature region, hydrocarbons are injected from
the hydrocarbon feed valve by the predetermined injection period
and, when the NO.sub.X which is stored in the exhaust purification
catalyst exceeds a predetermined second allowable value of a value
smaller than the first allowable value, the air-fuel ratio of the
exhaust gas which flows into the exhaust purification catalyst is
made rich.
Advantageous Effects of Invention
[0007] In the intermediate temperature region of the exhaust
purification catalyst, it is possible to obtain a higher NO.sub.X
purification rate than when the NO.sub.X removal action by the
first NO.sub.X removal method is being performed and than when the
NO.sub.X removal action by the second NO.sub.X removal method is
being performed.
BRIEF DESCRIPTION OF DRAWINGS
[0008] FIG. 1 is an overall view of a compression ignition type
internal combustion engine.
[0009] FIG. 2 is a view which schematically shows the surface part
of a catalyst carrier.
[0010] FIG. 3 is a view for explaining an oxidation reaction at an
exhaust purification catalyst.
[0011] FIG. 4 is a view which shows changes in an air-fuel ratio of
exhaust gas which flows into an exhaust purification catalyst.
[0012] FIG. 5 is a view which shows an NO.sub.X purification rate
R1.
[0013] FIGS. 6A and 6B are views for explaining an oxidation
reduction reaction in an exhaust purification catalyst.
[0014] FIGS. 7A and 7B are views for explaining an oxidation
reduction reaction in an exhaust purification catalyst.
[0015] FIG. 8 is a view which shows changes in an air-fuel ratio of
exhaust gas which flows into an exhaust purification catalyst.
[0016] FIG. 9 is a view which shows an NO.sub.X purification rate
R2.
[0017] FIG. 10 is a view which shows a relationship between a
vibration period .DELTA.T of hydrocarbon concentration and an
NO.sub.X purification rate R1.
[0018] FIGS. 11A and 11B are views which show maps of the injection
amount of hydrocarbons etc.
[0019] FIG. 12 is a view which shows an NO.sub.X release
control.
[0020] FIG. 13 is a view which shows a map of an exhausted NO.sub.X
amount NOXA.
[0021] FIG. 14 is a view which shows a fuel injection timing.
[0022] FIG. 15 is a view which shows a map of an additional
hydrocarbon feed amount WR.
[0023] FIG. 16 is a view which shows an NO.sub.X purification rate
R1 and an NO.sub.X purification rate R2.
[0024] FIGS. 17A and 17B are views for explaining the amount of
stored NO.sub.X in an exhaust purification catalyst.
[0025] FIG. 18 is a view which shows allowable values MAX and
SX.
[0026] FIG. 19 is a time chart which shows an NO.sub.X purification
control in the middle temperature region.
[0027] FIGS. 20A and 20B are views which show a map of the NO.sub.X
reducing rate RR etc.
[0028] FIG. 21 is a flow chart for performing an NO.sub.X
purification control.
[0029] FIG. 22 is a flow chart for performing an NO.sub.X
purification control.
[0030] FIG. 23 is a view for explaining the temperature
distribution in an exhaust purification catalyst.
DESCRIPTION OF EMBODIMENTS
[0031] FIG. 1 is an overall view of a compression ignition type
internal combustion engine.
[0032] Referring to FIG. 1, 1 indicates an engine body, 2 a
combustion chamber of each cylinder, 3 an electronically controlled
fuel injector for injecting fuel into each combustion chamber 2, 4
an intake manifold, and 5 an exhaust manifold. The intake manifold
4 is connected through an intake duct 6 to an outlet of a
compressor 7a of an exhaust turbocharger 7, while an inlet of the
compressor 7a is connected through an intake air amount detector 8
to an air cleaner 9. Inside the intake duct 6, a throttle valve 10
which is driven by an actuator is arranged. Around the intake duct
6, a cooling device 11 is arranged for cooling the intake air which
flows through the inside of the intake duct 6. In the embodiment
which is shown in FIG. 1, the engine cooling water is guided to the
inside of the cooling device 11 where the engine cooling water is
used to cool the intake air.
[0033] On the other hand, the exhaust manifold 5 is connected to an
inlet of an exhaust turbine 7b of the exhaust turbocharger 7, and
an outlet of the exhaust turbine 7b is connected through an exhaust
pipe 12 to an inlet of an exhaust purification catalyst 13. In an
embodiment of the present invention, this exhaust purification
catalyst 13 is comprised of an NO.sub.X storage catalyst 13. An
outlet of the exhaust purification catalyst 13 is connected to an
inlet of a particulate filter 14 and, upstream of the exhaust
purification catalyst 13 inside the exhaust pipe 12, a hydrocarbon
feed valve 15 is arranged for feeding hydrocarbons comprised of
diesel oil or other fuel used as fuel for a compression ignition
type internal combustion engine. In the embodiment shown in FIG. 1,
diesel oil is used as the hydrocarbons which are fed from the
hydrocarbon feed valve 15. Note that, the present invention can
also be applied to a spark ignition type internal combustion engine
in which fuel is burned under a lean air-fuel ratio. In this case,
from the hydrocarbon feed valve 15, hydrocarbons comprised of
gasoline or other fuel used as fuel of a spark ignition type
internal combustion engine are fed.
[0034] On the other hand, the exhaust manifold 5 and the intake
manifold 4 are connected with each other through an exhaust gas
recirculation (hereinafter referred to as an "EGR") passage 16.
Inside the EGR passage 16, an electronically controlled EGR control
valve 17 is arranged. Further, around the EGR passage 16, a cooling
device 18 is arranged for cooling the EGR gas which flows through
the inside of the EGR passage 16. In the embodiment which is shown
in FIG. 1, the engine cooling water is guided to the inside of the
cooling device 18 where the engine cooling water is used to cool
the EGR gas. On the other hand, each fuel injector 3 is connected
through a fuel feed tube 19 to a common rail 20. This common rail
20 is connected through an electronically controlled variable
discharge fuel pump 21 to a fuel tank 22. The fuel which is stored
inside of the fuel tank 22 is fed by the fuel pump 21 to the inside
of the common rail 20. The fuel which is fed to the inside of the
common rail 21 is fed through each fuel feed tube 19 to the fuel
injector 3.
[0035] An electronic control unit 30 is comprised of a digital
computer provided with a ROM (read only memory) 32, a RAM (random
access memory) 33, a CPU (microprocessor) 34, an input port 35, and
an output port 36, which are connected with each other by a
bidirectional bus 31. Downstream of the exhaust purification
catalyst 13, a temperature sensor 23 is arranged for detecting the
temperature of the exhaust gas flowing out from the exhaust
purification catalyst 13, and the output signals of this
temperature sensor 23 and intake air amount detector 8 are input
through respectively corresponding AD converters 37 to the input
port 35. Further, an accelerator pedal 40 has a load sensor 41
connected to it which generates an output voltage proportional to
the amount of depression L of the accelerator pedal 40. The output
voltage of the load sensor 41 is input through a corresponding AD
converter 37 to the input port 35. Furthermore, at the input port
35, a crank angle sensor 42 is connected which generates an output
pulse every time a crankshaft rotates by, for example, 15.degree..
On the other hand, the output port 36 is connected through
corresponding drive circuits 38 to each fuel injector 3, the
actuator for driving the throttle valve 10, hydrocarbon feed valve
15, EGR control valve 17, and fuel pump 21.
[0036] FIG. 2 schematically shows a surface part of a catalyst
carrier which is carried on a substrate of the exhaust purification
catalyst 13 shown in FIG. 1. At this exhaust purification catalyst
13, as shown in FIG. 2, for example, there is provided a catalyst
carrier 50 made of alumina on which precious metal catalysts 51
comprised of platinum Pt are carried. Furthermore, on this catalyst
carrier 50, a basic layer 53 is formed which includes at least one
element selected from potassium K, sodium Na, cesium Cs, or another
such alkali metal, barium Ba, calcium Ca, or another such alkali
earth metal, a lanthanide or another such rare earth and silver Ag,
copper Cu, iron Fe, iridium Ir, or another metal able to donate
electrons to NO.sub.X. In this case, on the catalyst carrier 50 of
the exhaust purification catalyst 13, in addition to platinum Pt,
rhodium Rh or palladium Pd may be further carried. Note that the
exhaust gas flows along the top of the catalyst carrier 50, so the
precious metal catalysts 51 can be said to be carried on the
exhaust gas flow surfaces of the exhaust purification catalyst 13.
Further, the surface of the basic layer 53 exhibits basicity, so
the surface of the basic layer 53 is called the "basic exhaust gas
flow surface parts 54".
[0037] If hydrocarbons are injected from the hydrocarbon feed valve
15 into the exhaust gas, the hydrocarbons are reformed by the
exhaust purification catalyst 13. In the present invention, at this
time, the reformed hydrocarbons are used to remove the NO.sub.X at
the exhaust purification catalyst 13. FIG. 3 schematically shows
the reformation action performed at the exhaust purification
catalyst 13 at this time. As shown in FIG. 3, the hydrocarbons HC
which are injected from the hydrocarbon feed valve 15 become
radical hydrocarbons HC with a small carbon number due to the
precious metal catalyst 51.
[0038] FIG. 4 shows the feed timing of hydrocarbons from the
hydrocarbon feed valve 15 and the change in the air-fuel ratio
(A/F) in of the exhaust gas which flows into the exhaust
purification catalyst 13. Note that, the change in the air-fuel
ratio (A/F) in depends on the change in concentration of the
hydrocarbons in the exhaust gas which flows into the exhaust
purification catalyst 13, so it can be said that the change in the
air-fuel ratio (A/F) in shown in FIG. 4 expresses the change in
concentration of the hydrocarbons. However, if the hydrocarbon
concentration becomes higher, the air-fuel ratio (A/F) in becomes
smaller, so, in FIG. 4, the more to the rich side the air-fuel
ratio (A/F) in becomes, the higher the hydrocarbon
concentration.
[0039] FIG. 5 shows the NO.sub.X purification rate R1 by the
exhaust purification catalyst 13 with respect to the catalyst
temperatures TC of the exhaust purification catalyst 13 when
periodically making the concentration of hydrocarbons which flow
into the exhaust purification catalyst 13 change so as to, as shown
in FIG. 4, periodically make the air-fuel ratio (A/F) in of the
exhaust gas flowing to the exhaust purification catalyst 13 rich.
In this regard, as a result of a research relating to NO.sub.X
purification for a long time, it is learned that if making the
concentration of hydrocarbons which flow into the exhaust
purification catalyst 13 vibrate by within a predetermined range of
amplitude and within a predetermined range of period, as shown in
FIG. 5, an extremely high NO.sub.X purification rate R1 is obtained
even in a 350.degree. C. or higher high temperature region.
[0040] Furthermore, it is learned that at this time, a large amount
of reducing intermediates which contain nitrogen and hydrocarbons
continues to be held or adsorbed on the surface of the basic layer
53, that is, on the basic exhaust gas flow surface parts 54 of the
exhaust purification catalyst 13, and the reducing intermediates
play a central role in obtaining a high NO.sub.X purification rate
R1. Next, this will be explained with reference to FIGS. 6A and 6B.
Note that, these FIGS. 6A and 6B schematically show the surface
part of the catalyst carrier 50 of the exhaust purification
catalyst 13. These FIGS. 6A and 6B show the reaction which is
presumed to occur when the concentration of hydrocarbons which flow
into the exhaust purification catalyst 13 is made to vibrate by
within a predetermined range of amplitude and within a
predetermined range of period.
[0041] FIG. 6A shows when the concentration of hydrocarbons which
flow into the exhaust purification catalyst 13 is low, while FIG.
6B shows when hydrocarbons are fed from the hydrocarbon feed valve
15 and the air-fuel ratio (A/F) in of the exhaust gas flowing to
the exhaust purification catalyst 13 is made rich, that is, the
concentration of hydrocarbons which flow into the exhaust
purification catalyst 13 becomes higher.
[0042] Now, as will be understood from FIG. 4, the air-fuel ratio
of the exhaust gas which flows into the exhaust purification
catalyst 13 is maintained lean except for an instant, so the
exhaust gas which flows into the exhaust purification catalyst 13
normally becomes a state of oxygen excess. At this time, part of
the NO which is contained in the exhaust gas deposits on the
exhaust purification catalyst 13, while part of the NO which is
contained in the exhaust gas, as shown in FIG. 6A, is oxidized on
the platinum 51 and becomes NO.sub.2. Next, this NO.sub.2 is
further oxidized and becomes NO.sub.3. Further, part of the
NO.sub.2 becomes NO.sub.2.sup.-. Therefore, on the platinum Pt 51,
NO.sub.2.sup.- and NO.sub.3 are produced. The NO which is deposited
on the exhaust purification catalyst 13 and the NO.sub.2.sup.- and
NO.sub.3 which are formed on the platinum Pt 51 are strong in
activity. Therefore, below, these NO, NO.sub.2.sup.-, and NO.sub.3
will be referred to as the "active NO.sub.X*".
[0043] On the other hand, if hydrocarbons are fed from the
hydrocarbon feed valve 15 and the air-fuel ratio (A/F) in of the
exhaust gas flowing to the exhaust purification catalyst 13 is made
rich, the hydrocarbons successively deposit over the entire exhaust
purification catalyst 13. The majority of the deposited
hydrocarbons successively react with oxygen and are burned. Part of
the deposited hydrocarbons are successively reformed and become
radicalized inside of the exhaust purification catalyst 13 as shown
in FIG. 3. Therefore, as shown in FIG. 6B, the hydrogen
concentration around the active NO.sub.X* becomes higher. In this
regard, if, after the active NO.sub.X* is produced, the state of a
high oxygen concentration around the active NO.sub.X* continues for
a constant time or more, the active NO.sub.X* is oxidized and is
absorbed in the form of nitrate ions NO.sub.3.sup.- inside the
basic layer 53. However, if, before this constant time elapses, the
hydrocarbon concentration around the active NO.sub.X* becomes
higher, as shown in FIG. 6B, the active NO.sub.X* reacts on the
platinum 51 with the radical hydrocarbons HC to thereby form the
reducing intermediates. The reducing intermediates are adhered or
adsorbed on the surface of the basic layer 53.
[0044] Note that, at this time, the first produced reducing
intermediate is considered to be a nitro compound R--NO.sub.2. If
this nitro compound R--NO.sub.2 is produced, the result becomes a
nitrile compound R--CN, but this nitrile compound R--CN can only
survive for an instant in this state, so immediately becomes an
isocyanate compound R--NCO. This isocyanate compound R--NCO becomes
an amine compound R--NH.sub.2 if hydrolyzed. However, in this case,
what is hydrolyzed is considered to be part of the isocyanate
compound R--NCO. Therefore, as shown in FIG. 6B, the majority of
the reducing intermediates which are held or adsorbed on the
surface of the basic layer 53 is believed to be the isocyanate
compound R--NCO and amine compound R--NH.sub.2.
[0045] On the other hand, as shown in FIG. 6B, if the produced
reducing intermediates are surrounded by the hydrocarbons HC, the
reducing intermediates are blocked by the hydrocarbons HC and the
reaction will not proceed any further. In this case, if the
concentration of hydrocarbons which flow into the exhaust
purification catalyst 13 is lowered and then the hydrocarbons which
are deposited around the reducing intermediates will be oxidized
and consumed, and thereby the concentration of oxygen around the
reducing intermediates becomes higher, the reducing intermediates
react with the NO.sub.X in the exhaust gas, react with the active
NO.sub.X*, react with the surrounding oxygen, or break down on
their own. Due to this, the reducing intermediates R--NCO and
R--NH.sub.2 are converted to N.sub.2, CO.sub.2, and H.sub.2O as
shown in FIG. 6A, therefore the NO.sub.X is removed.
[0046] In this way, in the exhaust purification catalyst 13, when
the concentration of hydrocarbons which flow into the exhaust
purification catalyst 13 is made higher, reducing intermediates are
produced, and after the concentration of hydrocarbons which flow
into the exhaust purification catalyst 13 is lowered, when the
oxygen concentration is raised, the reducing intermediates react
with the NO.sub.X in the exhaust gas or the active NO.sub.X* or
oxygen or break down on their own whereby the NO.sub.X is removed.
That is, in order for the exhaust purification catalyst 13 to
remove the NO.sub.X, the concentration of hydrocarbons which flow
into the exhaust purification catalyst 13 has to be periodically
changed.
[0047] Of course, in this case, it is necessary to raise the
hydrocarbon concentration to a concentration sufficiently high for
producing the reducing intermediates and it is necessary to lower
the hydrocarbon concentration to a concentration sufficiently low
for making the produced reducing intermediates react with the
NO.sub.X in the exhaust gas or the active NO.sub.X* or oxygen or
break down on their own. That is, it is necessary to make the
concentration of hydrocarbons which flow into the exhaust
purification catalyst 13 vibrate by within a predetermined range of
amplitude. Note that, in this case, it is necessary to hold these
reducing intermediates on the basic layer 53, that is, the basic
exhaust gas flow surface parts 54, until the produced reducing
intermediates R--NCO and R--NH.sub.2 react with the NO.sub.X in the
exhaust gas or the active NO.sub.X* or oxygen or break down
themselves. For this reason, the basic exhaust gas flow surface
parts 54 are provided.
[0048] On the other hand, if lengthening the feed period of the
hydrocarbons, the time until the oxygen concentration becomes
higher becomes longer in the period after the hydrocarbons are fed
until the hydrocarbons are next fed. Therefore, the active
NO.sub.X* is absorbed in the basic layer 53 in the form of nitrates
without producing reducing intermediates. To avoid this, it is
necessary to make the concentration of hydrocarbons which flow into
the exhaust purification catalyst 13 vibrate by within a
predetermined range of period.
[0049] Therefore, in the embodiment according to the present
invention, to react the NO.sub.X contained in the exhaust gas and
the reformed hydrocarbons and produce the reducing intermediates
R--NCO and R--NH.sub.2 containing nitrogen and hydrocarbons, the
precious metal catalysts 51 are carried on the exhaust gas flow
surfaces of the exhaust purification catalyst 13. To hold the
produced reducing intermediates R--NCO and R--NH.sub.2 inside the
exhaust purification catalyst 13, the basic exhaust gas flow
surface parts 54 are formed around the precious metal catalysts 51.
The reducing intermediates R--NCO and R--NH.sub.2 which are held on
the basic exhaust gas flow surface parts 54 are converted to
N.sub.2, CO.sub.2, and H.sub.2O. The vibration period of the
hydrocarbon concentration is made the vibration period required for
continuation of the production of the reducing intermediates R--NCO
and R--NH.sub.2. Incidentally, in the example shown in FIG. 4, the
injection interval is made 3 seconds.
[0050] If the vibration period of the hydrocarbon concentration,
that is, the injection period of hydrocarbons from the hydrocarbon
feed valve 15, is made longer than the, above predetermined range
of period, the reducing intermediates R--NCO and R--NH.sub.2
disappear from the surface of the basic layer 53. At this time, the
active NO.sub.X* which is produced on the platinum Pt 53, as shown
in FIG. 7A, diffuses in the basic layer 53 in the form of nitrate
ions NO.sub.3.sup.- and becomes nitrates. That is, at this time,
the NO.sub.X in the exhaust gas is absorbed in the form of nitrates
inside of the basic layer 53.
[0051] On the other hand, FIG. 7B shows the case where the air-fuel
ratio of the exhaust gas which flows into the exhaust purification
catalyst 13 is made the stoichiometric air-fuel ratio or rich when
the NO.sub.X is absorbed in the form of nitrates inside of the
basic layer 53. In this case, the oxygen concentration in the
exhaust gas falls, so the reaction proceeds in the opposite
direction (NO.sub.3.sup.-.fwdarw.NO.sub.2), and consequently the
nitrates absorbed in the basic layer 53 successively become nitrate
ions NO.sub.3.sup.- and, as shown in FIG. 7B, are released from the
basic layer 53 in the form of NO.sub.2. Next, the released NO.sub.2
is reduced by the hydrocarbons HC and CO contained in the exhaust
gas.
[0052] FIG. 8 shows the case of making the air-fuel ratio (A/F) in
of the exhaust gas which flows into the exhaust purification
catalyst 13 temporarily rich slightly before the NO.sub.X
absorption ability of the basic layer 53 becomes saturated. Note
that, in the example shown in FIG. 8, the time interval of this
rich control is 1 minute or more. In this case, the NO.sub.X which
was absorbed in the basic layer 53 when the air-fuel ratio (A/F) in
of the exhaust gas was lean is released all at once from the basic
layer 53 and reduced when the air-fuel ratio (A/F) in of the
exhaust gas is made temporarily rich. Therefore, in this case, the
basic layer 53 plays the role of an absorbent for temporarily
absorbing NO.sub.X.
[0053] Note that, at this time, sometimes the basic layer 53
temporarily adsorbs the NO.sub.X. Therefore, if using term of
"storage" as a term including both "absorption" and "adsorption",
at this time, the basic layer 53 performs the role of an NO.sub.X
storage agent for temporarily storing the NO.sub.X. That is, in
this case, if the ratio of the air and fuel (hydrocarbons) which
are supplied into the engine intake passage, combustion chambers 2,
and upstream of the exhaust purification catalyst 13 in the exhaust
passage is referred to as "the air-fuel ratio of the exhaust gas",
the exhaust purification catalyst 13 functions as an NO.sub.X
storage catalyst which stores the NO.sub.X when the air-fuel ratio
of the exhaust gas is lean and releases the stored NO.sub.X when
the oxygen concentration in the exhaust gas falls.
[0054] FIG. 9 shows the NO.sub.X purification rate R2 when making
the exhaust purification catalyst 13 function as an NO.sub.X
storage catalyst in this way. Note that, the abscissa of the FIG. 9
shows the catalyst temperature TC of the exhaust purification
catalyst 13. When making the exhaust purification catalyst 13
function as an NO.sub.X storage catalyst, as shown in FIG. 9, when
the catalyst temperature TC is 250.degree. C. to 300.degree. C., an
extremely high NO.sub.X purification rate is obtained, but when the
catalyst temperature TC becomes a 350.degree. C. or higher high
temperature, the NO.sub.X purification rate R2 falls.
[0055] In this way, when the catalyst temperature TC becomes
350.degree. C. or more, the NO.sub.X purification rate R2 falls
because if the catalyst temperature TC becomes 350.degree. C. or
more, NO.sub.X is less easily stored and the nitrates break down by
heat and are released in the form of NO.sub.2 from the exhaust
purification catalyst 13. That is, so long as storing NO.sub.X in
the form of nitrates, when the catalyst temperature TC is high, it
is difficult to obtain a high NO.sub.X purification rate R2.
However, in the new NO.sub.X purification method shown from FIG. 4
to FIGS. 6A and 6B, nitrates are not formed or even if formed are
extremely small in amount, and consequently, as shown in FIG. 5,
even when the catalyst temperature TC is high, a high NO.sub.X
purification rate R1 is obtained.
[0056] In the embodiment according to the present invention, to be
able to purify NO.sub.X by using this new NO.sub.X purification
method, a hydrocarbon feed valve 15 for feeding hydrocarbons is
arranged in the engine exhaust passage, an exhaust purification
catalyst 13 is arranged in the engine exhaust passage downstream of
the hydrocarbon feed valve 15, precious metal catalysts 51 are
carried on the exhaust gas flow surfaces of the exhaust
purification catalyst 13, basic exhaust gas flow surface parts 54
are formed around the precious metal catalysts 51, the exhaust
purification catalyst 13 has the property of reducing the NO.sub.X
which is contained in exhaust gas if the concentration of
hydrocarbons which flow into the exhaust purification catalyst 13
is made to vibrate by within a predetermined range of amplitude and
within a predetermined range of period and has the property of
being increased in storage amount of NO.sub.X which is contained in
exhaust gas if the vibration period of the hydrocarbon
concentration is made longer than this predetermined range, and, at
the time of engine operation, the hydrocarbons are injected from
the hydrocarbon feed valve 15 within the predetermined range of
period to thereby reduce the NO.sub.X which is contained in the
exhaust gas in the exhaust purification catalyst 13.
[0057] That is, the NO.sub.X purification method which is shown
from FIG. 4 to FIGS. 6A and 6B can be said to be a new NO.sub.X
purification method designed to remove NO.sub.X without forming so
much nitrates in the case of using an exhaust purification catalyst
which carries precious metal catalysts and forms a basic layer
which can absorb NO.sub.X. In actuality, when using this new
NO.sub.X purification method, the nitrates which are detected from
the basic layer 53 become extremely smaller in amount compared with
the case where making the exhaust purification catalyst 13 function
as an NO.sub.X storage catalyst. Note that, this new NO.sub.X
purification method will be referred to below as the "first
NO.sub.X purification method".
[0058] Now, as mentioned before, if the injection period .DELTA.T
of the hydrocarbons from the hydrocarbon feed valve 15 becomes
longer, the time period in which the oxygen concentration around
the active NO.sub.X* becomes higher becomes longer in the time
period after the hydrocarbons are injected to when the hydrocarbons
are next injected. In this case, in the embodiment shown in FIG. 1,
if the injection period .DELTA.T of the hydrocarbons becomes longer
than about 5 seconds, the active NO.sub.X* starts to be absorbed in
the form of nitrates inside the basic layer 53. Therefore, as shown
in FIG. 10, if the vibration period .DELTA.T of the hydrocarbon
concentration becomes longer than about 5 seconds, the NO.sub.X
purification rate R1 falls. Therefore, the injection period
.DELTA.T of the hydrocarbons has to be made 5 seconds or less.
[0059] On the other hand, in the embodiment of the present
invention, if the injection period .DELTA.T of the hydrocarbons
becomes about 0.3 second or less, the injected hydrocarbons start
to build up on the exhaust gas flow surfaces of the exhaust
purification catalyst 13, therefore, as shown in FIG. 10, if the
injection period .DELTA.T of the hydrocarbons becomes about 0.3
second or less, the NO.sub.X purification rate R1 falls. Therefore,
in the embodiment according to the present invention, the injection
period of the hydrocarbons is made from 0.3 second to 5
seconds.
[0060] Now, in the embodiment according to the present invention,
when the NO.sub.X purification action by the first NO.sub.X
purification method is performed, by controlling the injection
amount and injection timing of hydrocarbons from the hydrocarbon
feed valve 15, the air-fuel ratio (A/F) in of the exhaust gas
flowing into the exhaust purification catalyst 13 and the injection
period .DELTA.T of the hydrocarbons are controlled so as to become
the optimal values for the engine operating state. In this case, in
the embodiment according to the present invention, the optimum
hydrocarbon injection amount WT when the NO.sub.X purification
action by the first NO.sub.X purification method is performed is
stored as a function of the injection amount Q from fuel injectors
3 and the engine speed N in the form of a map such as shown in FIG.
11A in advance in the ROM 32. Further, the optimum injection period
.DELTA.T of the hydrocarbons at this time is also stored as a
function of the injection amount Q from the fuel injectors 3 and
the engine speed N in the form of a map such as shown in FIG. 11B
in advance in the ROM 32.
[0061] Next, referring to FIG. 12 to FIG. 15, an NO.sub.X
purification method when making the exhaust purification catalyst
13 function as an NO.sub.X storage catalyst will be explained
specifically. The NO.sub.X purification method in the case of
making the exhaust purification catalyst 13 function as an NO.sub.X
storage catalyst in this way will be referred to below as the
"second NO.sub.X purification method".
[0062] In this second NO.sub.X purification method, as shown in
FIG. 12, when the stored NO.sub.X amount .SIGMA.NO.sub.X of
NO.sub.X which is stored in the basic layer 53 exceeds a first
predetermined allowable amount MAX 1, the air-fuel ratio (A/F) in
of the exhaust gas flowing into the exhaust purification catalyst
13 is temporarily made rich. If the air-fuel ratio (A/F) in of the
exhaust gas is made rich, the NO.sub.X which was stored in the
basic layer 53 when the air-fuel ratio (A/F) in of the exhaust gas
was lean is released from the basic layer 53 all at once and
reduced. Due to this, the NO.sub.X is removed.
[0063] The stored NO.sub.X amount .SIGMA.NO.sub.X is, for example,
calculated from the amount of NO.sub.X which is exhausted from the
engine. In this embodiment according to the present invention, the
exhausted NO.sub.X amount NOXA of NO.sub.X which is exhausted from
the engine per unit time is stored as a function of the injection
amount Q and engine speed N in the form of a map such as shown in
FIG. 13 in advance in the ROM 32. The stored NO.sub.X amount
.SIGMA.NO.sub.X is calculated from this exhausted NO.sub.X amount
NOXA. In this case, as explained before, the period at which the
air-fuel ratio (A/F) in of the exhaust gas is made rich is usually
1 minute or more.
[0064] In this second NO.sub.X purification method, as shown in
FIG. 14, by injecting an additional fuel WR into each combustion
chamber 2 from the fuel injector 3 in addition to the
combustion-use fuel Q, the air-fuel ratio (A/F) in of the exhaust
gas which flows into the exhaust purification catalyst 13 is made
rich. Note that, in FIG. 14, the abscissa indicates the crank
angle. This additional fuel WR is injected at a timing at which it
will burn, but will not appear as engine output, that is, slightly
before ATDC90.degree. after compression top dead center. This fuel
amount WR is stored as a function of the injection amount Q and
engine speed N in the form of a map such as shown in FIG. 15 in
advance in the ROM 32. In this way, in case where the second
NO.sub.X purification method is performed, when the air-fuel ratio
(A/F) in of the exhaust gas flowing into the exhaust purification
catalyst 13 should be made rich, the air-fuel ratio (A/F) in of the
exhaust gas discharged from the combustion chamber 2 is made rich
by feeding the additional fuel WR to the combustion chamber 2.
[0065] FIG. 16 shows together the NO.sub.X removal rate R1 when the
NO.sub.X removal action by the first NO.sub.X removal method is
being performed and the NO.sub.X removal rate R2 when the NO.sub.X
removal action by the second NO.sub.X removal method is being
performed.
[0066] As shown in FIG. 16, the NO.sub.X removal rate R1 when the
NO.sub.X removal action by the first NO.sub.X removal method is
being performed becomes extremely high when the catalyst
temperature TC is 350.degree. C. or more and falls along with the
fall of the catalyst temperature TC when the catalyst temperature
becomes 350.degree. C. or less. On the other hand, the NO.sub.X
removal rate R2 when the NO.sub.X removal action by the second
NO.sub.X removal method is being performed becomes extremely high
when the catalyst temperature TC is 250.degree. C. to 300.degree.
C., starts to fall a bit at a time along with the catalyst
temperature becoming higher when the catalyst temperature TC
becomes 300.degree. C. or more, and rapidly falls along with the
rise of the catalyst temperature TC when the catalyst temperature
TC becomes 350.degree. C. or more.
[0067] In FIG. 16, T1 shows the catalyst temperature when the
NO.sub.X removal rate R2 starts to fall when the catalyst
temperature TC rises in the case where the NO.sub.X removal action
by the second NO.sub.X removal method is being performed, while T2
shows the catalyst temperature when the NO.sub.X removal rate R2
becomes zero when the catalyst temperature TC further rises in the
case where the NO.sub.X removal action by the second NO.sub.X
removal method is being performed. In this embodiment according to
the present invention, the temperature region where the catalyst
temperature TC is the temperature T1 or less is called the "low
temperature region", the temperature region where the catalyst
temperature TC is between the temperature T1 and the temperature T2
is called the "intermediate temperature region", and the
temperature region where the catalyst temperature TC is the
temperature T2 or more is called the "high temperature region".
Therefore, in this embodiment according to the present invention,
the intermediate temperature region shows the temperature range
where the NO.sub.X removal rate R2 continues to fall if the
temperature TC of the exhaust purification catalyst 13 rises in the
case where the NO.sub.X removal action by the second NO.sub.X
removal method is being performed.
[0068] As shown in FIG. 16, in the high temperature region where
the catalyst temperature TC is higher than the temperature T2, the
NO.sub.X removal rate R2 becomes zero. With the second NO.sub.X
removal method, NO.sub.X cannot be removed. Therefore, in this
embodiment according to the present invention, at this time, that
is, in the high temperature region, the NO.sub.X removal action by
the first NO.sub.X removal method is performed. On the other hand,
in the low temperature region where the catalyst temperature TC is
lower than the temperature T1, the NO.sub.X removal rate R2 becomes
high. Therefore, in this embodiment according to the present
invention, at this time, that is, in the low temperature region,
the NO.sub.X removal action by the second NO.sub.X removal method
is performed. As opposed to this, when the catalyst temperature TC
is between the temperature T1 and the temperature T2, that is, in
the intermediate temperature region, the NO.sub.X removal rate R1
falls in part of the temperature region and the NO.sub.X removal
rate R2 falls in a considerably broad temperature region.
Therefore, in this case, even if using either of the first and the
second NO.sub.X removal methods, the NO.sub.X removal rate will
fall in some temperature region.
[0069] Therefore, in the present invention, when the catalyst
temperature TC is in the intermediate temperature region, the first
NO.sub.X removal method and the second NO.sub.X removal method are
jointly used so as to obtain an NO.sub.X removal rate which is
higher than the NO.sub.X removal rate R1 when the NO.sub.X removal
action by the first NO.sub.X removal method is performed and the
NO.sub.X removal rate R2 when the NO.sub.X removal action by the
second NO.sub.X removal method is performed. Next, this will be
explained with reference to FIGS. 17A and 17B. Note that, FIGS. 17A
and 17B show the exhaust purification catalyst 13. In FIGS. 17A and
17B, X shows when the exhaust purification catalyst 13 is not
storing NO.sub.X. On the other hand, in FIGS. 17A and 17B, Y shows
when the exhaust purification catalyst 13 is storing NO.sub.X, and
the hatchings in FIGS. 17A and 17B show the ratio of the NO.sub.X
which is actually stored with respect to the total amount of
NO.sub.X which the exhaust purification catalyst 13 can store, that
is, the NO.sub.X storage ratio.
[0070] FIG. 17A shows when the NO.sub.X removal action by the
second NO.sub.X removal method is being performed. At this time,
the state which is shown by X and the state which is shown by Y are
repeated. That is, at this time, as shown in FIG. 17A by Y, if the
NO.sub.X amount which is stored in the exhaust purification
catalyst 13 approaches saturation, that is, exceeds the first
allowable value MAX which is shown in FIG. 12, the air-fuel ratio
of the exhaust gas which flows into the exhaust purification
catalyst 13 is made rich. Due to this, as shown in FIG. 17A by X,
the NO.sub.X storage ratio in the exhaust purification catalyst 13
is made zero. Next, again, the amount of NO.sub.X stored in the
exhaust purification catalyst 13 increases.
[0071] As opposed to this, FIG. 17B shows the case where the first
NO.sub.X removal method and the second NO.sub.X removal method are
jointly used in the intermediate temperature region. In this case,
in FIG. 17B, the state which is shown by X and the state which is
shown by Y are repeated. That is, at this time, as shown in FIG.
17B by Y, if the NO.sub.X amount which is stored in the exhaust
purification catalyst 13 becomes the second allowable value SX
which is smaller than the first allowable value MAX, that is, in
the example which is shown in FIG. 17B, if the NO.sub.X storage
ratio becomes 50 percent, the air-fuel ratio of the exhaust gas
which flows into the exhaust purification catalyst 13 is made rich.
Due to this, as shown in FIG. 17B by X, the NO.sub.X storage ratio
in the exhaust purification catalyst 13 is made zero. Next, again,
the amount of NO.sub.X which is stored in the exhaust purification
catalyst 13 increases. In this way, in the example which is shown
in FIG. 17B, this second allowable value SX is made the stored
NO.sub.X amount when the NO.sub.X storage ratio is 50 percent.
[0072] That is, if the exhaust purification catalyst 13 stores
NO.sub.X when the NO.sub.X removal action by the first NO.sub.X
removal method is being performed, NO.sub.X becomes harder to stick
to or be adsorbed in the form of a reducing intermediate at the
surface part of the basicity layer 53 where the NO.sub.X is stored.
Therefore, if the amount of NO.sub.X which is stored at the exhaust
purification catalyst 13 increases, the amount of NO.sub.X which
can be removed by the NO.sub.X removal action by the first NO.sub.X
removal method decreases. Therefore, to remove NO.sub.X well by
using the NO.sub.X removal action by the first NO.sub.X removal
method even if the exhaust purification catalyst 13 stores
NO.sub.X, it is necessary to prevent the exhaust purification
catalyst 13 from storing a large amount of NO.sub.X. In this case,
compared with the maximum stored NO.sub.X amount when the NO.sub.X
removal action by the second NOx removal method is being performed,
which is shown in FIG. 17A by Y, if limiting the maximum stored
NO.sub.X amount when the NO.sub.X removal action by the first
NO.sub.X removal method is being performed to a small amount such
as shown in FIG. 17B by Y, it is possible to sufficiently secure an
area of the surface part of the basicity layer 53 at which the
reducing intermediate can easily stick or be adsorbed and,
therefore, when the NO.sub.X removal action by the first NO.sub.X
removal method is performed, a good NO.sub.X removal action is
performed.
[0073] Therefore, in the example which is shown in FIG. 17B, in the
intermediate temperature region, to limit the maximum stored
NO.sub.X amount to a small amount such as shown in FIG. 17B by Y,
when the stored NO.sub.X amount in the exhaust purification
catalyst 13 becomes the second allowable value SX which is smaller
than the first allowable value MAX, the air-fuel ratio of the
exhaust gas which flows into the exhaust purification catalyst 13
is made rich. That is, in the intermediate temperature region, the
NO.sub.X removal action by the first NO.sub.X removal method is
performed, and when the stored NO.sub.X amount in the exhaust
purification catalyst 13 becomes the second allowable value SX, the
air-fuel ratio of the exhaust gas which flows into the exhaust
purification catalyst 13 is made rich. By doing this, if jointly
using the first NO.sub.X removal method and the second NO.sub.X
removal method, the NO.sub.X removal action by the second NO.sub.X
removal method is superposed in form over the NO.sub.X removal
action by the first NO.sub.X removal method, so a high NO.sub.X
removal rate such as shown in FIG. 16 by R is obtained.
[0074] Therefore, in the present invention, there is provided an
exhaust purification system of an internal combustion engine in
which an exhaust purification catalyst 13 is arranged in an engine
exhaust passage and a hydrocarbon feed valve 15 is arranged in the
engine exhaust passage upstream of the exhaust purification
catalyst 13, a precious metal catalyst 51 is carried on an exhaust
gas flow surface of the exhaust purification catalyst 13 and a
basic exhaust gas flow surface part 54 is formed around the
precious metal catalyst 51, the exhaust purification catalyst 13
has the property of reducing the NO.sub.X which is contained in
exhaust gas if making a concentration of hydrocarbons which flow
into the exhaust purification catalyst 13 vibrate by within a
predetermined range of amplitude and by within a predetermined
range of period and has the property of being increased in storage
amount of NO.sub.X which is contained in the exhaust gas if making
the vibration period of the hydrocarbon concentration longer than
the predetermined range, a first NO.sub.X removal method which
injects hydrocarbons from the hydrocarbon feed valve 15 by a
predetermined injection period to thereby remove the NO.sub.X which
is contained in the exhaust gas and a second NO.sub.X removal
method which makes the air-fuel ratio of the exhaust gas which
flows into the exhaust purification catalyst 13 rich to make the
exhaust purification catalyst 13 release the stored NO.sub.X when
the NO.sub.X which is stored in the exhaust purification catalyst
13 exceeds a predetermined first allowable value MAX are
selectively used, and in the second NO.sub.X removal method, the
period by which the air-fuel ratio of the exhaust gas which flows
into the exhaust purification catalyst 13 is made rich is longer
than the above-mentioned predetermined injection period, in which
exhaust purification system of an internal combustion engine,
temperature regions which the exhaust purification catalyst can
take at the time of engine operation are divided into the three
regions of a low temperature region, an intermediate temperature
region, and a high temperature region, in the high temperature
region, an NO.sub.X removal action by the first NO.sub.X removal
method is performed, in the low temperature region, an NO.sub.X
removal action by the second NO.sub.X removal method is performed,
and in the intermediate temperature region, hydrocarbons are
injected from the hydrocarbon feed valve 15 by the predetermined
injection period and, when the NO.sub.X which is stored in the
exhaust purification catalyst 13 exceeds a predetermined second
allowable value SX of a value smaller than the first allowable
value MAX, the air-fuel ratio of the exhaust gas which flows into
the exhaust purification catalyst 13 is made rich.
[0075] FIG. 18 shows an embodiment in which the second allowable
value SX is changed in accordance with the temperature TC of the
exhaust purification catalyst 13. Note that, FIG. 18 also shows the
changes in the NO.sub.X removal rates R1, R2, and R. Now then, in
the intermediate temperature region, if the catalyst temperature TC
becomes higher, the amount of NO.sub.X which can be stored in the
exhaust purification catalyst 13 becomes smaller. When the amount
of NO.sub.X which can be stored in the exhaust purification
catalyst 13 becomes smaller, unless the exhaust purification
catalyst 13 is made to release NO.sub.X while the amount of
NO.sub.X which is stored in the exhaust purification catalyst 13 is
small, NO.sub.X can no longer be stored. Therefore, when the amount
of NO.sub.X which can be stored in the exhaust purification
catalyst 13 becomes small, it is necessary to make the air-fuel
ratio of the exhaust gas which flows into the exhaust purification
catalyst 13 rich while the amount of storage of NO.sub.X is small.
Therefore, in the embodiment which is shown in FIG. 19, the second
allowable value SX is made smaller as the temperature TC of the
exhaust purification catalyst 13 becomes higher. If the amount of
NO.sub.X which can be stored in the exhaust purification catalyst
13 becomes smaller, the amount of NO.sub.X which can be removed by
the NO.sub.X removal action by the second NO.sub.X removal method
is decreased and the amount of NO.sub.X which is removed by the
NO.sub.X removal action by the first NO.sub.X removal method is
increased. That is, in the embodiment which is shown in FIG. 19, in
the intermediate temperature region, as the temperature TC of the
exhaust purification catalyst 13 becomes higher, the amount of
NO.sub.X which is removed by the NO.sub.X removal action by the
second NO.sub.X removal method is decreased and the amount of
NO.sub.X which is removed by the NO.sub.X removal action by the
first NO.sub.X removal method is increased.
[0076] FIG. 19 shows a time chart of NO.sub.X purification control
in the intermediate temperature region. Note that, FIG. 19 shows
the hydrocarbon feed signal from the hydrocarbon feed valve 15, the
feed signal of the additional fuel WR from the fuel injector 3, the
change in the NO.sub.X amount .SIGMA.NOX which is stored in the
exhaust purification catalyst 13, and the change in the air-fuel
ratio (A/F) in of the exhaust gas which flows into the exhaust
purification catalyst 13. Further, FIG. 18 shows the first
allowable value MAX and the second allowable value SX. From FIG.
18, it will be understood that the second allowable value SX is
considerably smaller compared with the first allowable value
MAX.
[0077] As will be understood from FIG. 19, when the stored NO.sub.X
amount .SIGMA.NOX is smaller than the second allowable value SX,
hydrocarbons are injected in accordance with the hydrocarbon feed
signal from the hydrocarbon feed valve 15 by a predetermined
injection period and the NO.sub.X removal action by the first
NO.sub.X removal method is performed. As opposed to this, when the
stored NO.sub.X amount .SIGMA.NOX exceeds the second allowable
value SX, additional fuel WR is injected from the fuel injector 3
over a certain time period in accordance with the additional fuel
feed signal and thereby the air-fuel ratio (A/F) in of the exhaust
gas which flows into the exhaust purification catalyst 13 is made
rich. When the additional fuel WR finishes being injected, the
stored NO.sub.X finishes being released, so the stored NO.sub.X
amount .SIGMA.NOX becomes zero. Note that, when additional fuel WR
is injected from the fuel injector 3 and the air-fuel ratio (A/F)
in of the exhaust gas which flows into the exhaust purification
catalyst 13 becomes rich, if hydrocarbons are injected from the
hydrocarbon feed valve 15, the air-fuel ratio (A/F) in of the
exhaust gas which flows into the exhaust purification catalyst 13
will become too rich, the hydrocarbons will slip through the
exhaust purification catalyst 13, and there is the danger of white
smoke being generated. Therefore, as shown in FIG. 18 by PH, in the
intermediate temperature region, while the feed of additional fuel
WR from the fuel injector 3 is being used to make the air-fuel
ratio of the exhaust gas which flows into the exhaust purification
catalyst 13 rich, injection of hydrocarbons from the hydrocarbon
feed valve 15 is suspended.
[0078] On the other hand, in this embodiment according to the
present invention, when the NO.sub.X removal action by the first
NO.sub.X removal method is being performed, the NO.sub.X amount NOX
which is stored per unit time in the exhaust purification catalyst
13 is calculated based on the following formula:
NOX=(NOXA-RR)KR
Here, NOXA shows the amount of NO.sub.X which is exhausted per unit
time from the engine which is shown in FIG. 13, RR shows the amount
of reduction of NO.sub.X per unit time by hydrocarbons which are
injected from the hydrocarbon feed valve 15, and KR shows the
storage rate of NO.sub.X in the exhaust purification catalyst 13.
As shown in FIGS. 11A and 11B, the injection amount WT and
injection period .DELTA.T of hydrocarbons from the hydrocarbon feed
valve 15 are determined in advance in accordance with the operating
state of the engine. Therefore, the NO.sub.X reduction amount RR
which is reduced per unit time of injection by the hydrocarbons
which are injected from the hydrocarbon feed valve 15 is also
determined in advance in accordance with the operating state of the
engine. Therefore, in this embodiment according to the present
invention, this NO.sub.X reduction amount per unit time is stored
as a function of the amount of injection Q from the fuel injector 3
and the engine speed N in the form of a map such as shown in FIG.
20A in advance in the ROM 32. On the other hand, the NO.sub.X
storage rate KR shows the ratio of the amount of NO.sub.X which is
stored in the exhaust purification catalyst 13 in the amount of
NO.sub.X (NOXA-RR) which could not be reduced by the hydrocarbons
injected from the hydrocarbon feed valve 15. This NO.sub.X storage
rate KP, as shown in FIG. 20B, falls as the temperature TC of the
exhaust purification catalyst 13 becomes higher.
[0079] If the NO.sub.X amount NOX which is stored in the exhaust
purification catalyst 13 per unit time is calculated, the NO.sub.X
amount .SIGMA.NOX which is stored in the exhaust purification
catalyst 13 is calculated by cumulatively adding this NO.sub.X
amount NOX. In this way, in this embodiment according to the
present invention, in the intermediate temperature region, when the
NO.sub.X removal action by the first NO.sub.X removal method is
being performed, the NO.sub.X amount .SIGMA.NOX which is stored in
the exhaust purification catalyst 13 is calculated from the
NO.sub.X amount NOXA which is exhausted from the engine, the
NO.sub.X reduction amount RR which is determined from the operating
state of the engine, and the NO.sub.X storage rate KP which is
determined from the temperature TC of the exhaust purification
catalyst 13 and, when the calculated NO.sub.X amount .SIGMA.NOX
exceeds the second allowable value SX, the air-fuel ratio of the
exhaust gas which flows into the exhaust purification catalyst 13
is made rich.
[0080] FIG. 21 and FIG. 22 show the NO.sub.X purification control
routine for performing the NO.sub.X removal method which is shown
in FIG. 19. This routine is executed by interruption every certain
time period.
Referring to FIG. 21, first, to start, at step 60, the detected
value of the temperature sensor 23 is used as the basis to
calculate the temperature TC of the exhaust purification catalyst
13. Next, at step 61, it is judged if the catalyst temperature TC
is lower than the temperature T1. When the catalyst temperature TC
is lower than the temperature T1, that is, when in the low
temperature region, it is judged that the NO.sub.X removal action
by the second NO.sub.X removal method should be performed. The
routine proceeds to step 62 where the NO.sub.X removal action by
the second NO.sub.X removal method is performed.
[0081] That is, at step 62, the NO.sub.X amount NOXA which is
exhausted per unit time is calculated from the map which is shown
in FIG. 13. Next at step 63, the exhausted NO.sub.X amount NOXA is
added to .SIGMA.NOX to calculate the stored NO.sub.X amount
.SIGMA.NOX. Next, at step 64, it is judged if the stored NO.sub.X
amount .SIGMA.NOX exceeds the first allowable value MAX. When
.SIGMA.NOX>MAX, the routine proceeds to step 65 where the amount
of additional fuel WR is calculated from the map which is shown in
FIG. 15. Next, at step 66, the action of injection of additional
fuel is performed. At this time, the air-fuel ratio (A/F) in of the
exhaust gas which flows into the exhaust purification catalyst 13
is made rich. Next, at step 67, it is judged if the exhaust
purification catalyst 13 has finished being regenerated. When it is
judged that the exhaust purification catalyst 13 has finished being
regenerated, the routine proceeds to step 68 where .SIGMA.NOX is
cleared.
[0082] On the other hand, at step 61, when it is judged that the
exhaust temperature TC is higher than the temperature T1, the
routine proceeds to step 69 where it is judged if the catalyst
temperature TC is higher than the temperature T2. When the catalyst
temperature T is higher than the temperature T2, that is, when in
the high temperature region, it is judged that the NO.sub.X removal
action by the first NO.sub.X removal method should be performed and
the routine proceeds to step 70 where the NO.sub.X removal action
by the first NO.sub.X removal method is performed. That is, at step
70, the injection period .DELTA.T of hydrocarbons is read from FIG.
11B. Next, at step 71, it is judged if the injection timing has
been reached. When the injection timing has been reached, the
routine proceeds to step 72 where the amount of injection WT of
hydrocarbons is calculated from FIG. 11A. Next, at step 73, the
injection amount WT which is calculated at step 72 is used to
inject hydrocarbons from the hydrocarbon feed valve 15.
[0083] On the other hand, when it is judged at step 69 that the
catalyst temperature TC is lower than the temperature T2, that is,
when it is the intermediate temperature region, the routine
proceeds to step 74 of FIG. 22 where it is judged if the injection
prohibit flag which shows that injection of hydrocarbons from the
hydrocarbon feed valve 15 should be prohibited has been set. When
the injection prohibit flag is not set, the routine proceeds to
step 75 where the NO.sub.X amount NOXA which is exhausted per unit
time is calculated from the map which is shown in FIG. 13, the
NO.sub.X reduction amount RR is calculated from the map which is
shown in FIG. 20A, and the NO.sub.X storage rate KP is calculated
from FIG. 20B. Next, at step 76, the NO.sub.X amount NOX which is
stored per unit time is calculated based on the following
formula:
NOX=(NOXA-RR)KR
Next at step 77, the NO.sub.X amount .SIGMA.NOX which is stored in
the exhaust purification catalyst 13 is calculated based on the
following formula:
.SIGMA.NOX=.SIGMA.NOX+NOX
[0084] Next, at step 78, the second allowable value SX which is
shown in FIG. 18 is calculated. Next, at step 79, it is judged if
the stored NO.sub.X amount .SIGMA.NOX exceeds the second allowable
value SX. When the stored NO.sub.X amount .SIGMA.NOX does not
exceed the second allowable value SX, the routine proceeds to step
80 where the NO.sub.X removal action by the first NO.sub.X removal
method is performed. That is, at step 80, the injection period
.DELTA.T of the hydrocarbons is read from FIG. 11B. Next, at step
81, it is judged if the injection timing has been reached. When the
injection timing has been reached, the routine proceeds to step 82
where the injection amount WT of hydrocarbons is calculated from
FIG. 11A. Next, at step 83, the injection amount WT which is
calculated at step 82 is used to injection hydrocarbons from the
hydrocarbon feed valve 15. Next, at step 84, the NO.sub.X amount
CNO which is released from the exhaust purification catalyst 13 at
the time of injection of hydrocarbons from the hydrocarbon feed
valve 15, is subtracted from the stored NO.sub.X amount
.SIGMA.NOX.
[0085] On the other hand, when it is judged at step 79 that the
stored NO.sub.X amount .SIGMA.NOX exceeds the second allowable
value SX, the routine proceeds to step 85 where the injection
prohibit flag is set, next the routine proceeds to step 86. If the
injection prohibit flag is set, at the next processing cycle, the
routine jumps from step 74 to step 86. At step 86, the amount of
additional fuel WRL which is required for making the stored
NO.sub.X be released is calculated. Next, at step 87, the action of
injection of additional fuel to the inside of the combustion
chamber 2 is performed. At this time, the air-fuel ratio (A/F) in
of the exhaust gas which flows into the exhaust purification
catalyst 13 is made rich. Next, at step 88, it is judged if the
exhaust purification catalyst 13 has finished being regenerated.
When it is judged that the exhaust purification catalyst 13 has
finished being regenerated, the routine proceeds to step 89 where
the injection prohibit flag is reset. Next at step 90, .SIGMA.NOX
is cleared.
[0086] In this regard, when the catalyst temperature TC is
maintained at the intermediate temperature region, a large
temperature difference does not arise between the upstream side and
the downstream side of the exhaust purification catalyst 13. On the
other hand, for example, if the amount of injection of hydrocarbons
from the hydrocarbon feed valve 15 for regeneration of the
particulate filter is increased, the temperature TC of the exhaust
purification catalyst 13 becomes higher and the catalyst
temperature TC shifts from the intermediate temperature region to
the high temperature region. Next, if the particulate filter
finishes being regenerated, the catalyst temperature TC falls and
the catalyst temperature TC again becomes the intermediate
temperature region. In this regard, when the particulate filter
finishes being regenerated and the catalyst temperature TC falls,
the exhaust purification catalyst 13 is cooled from the upstream
side. Therefore, at this time, as shown in FIG. 23, the downstream
side becomes higher than the upstream side. That is, when the
particulate filter finishes being regenerated, a large temperature
difference arises between the upstream side and the downstream side
of the exhaust purification catalyst 13. At this time, the catalyst
temperature TC which is calculated from the detected value of the
temperature sensor 23 becomes the mean temperature such as shown by
Tm in FIG. 23.
[0087] On the other hand, as explained above, in the intermediate
temperature region, the NO.sub.X amount NOX which is stored per
unit time is calculated based on the following formula:
NOX=(NOXA-RR)KR
The NO.sub.X reduction amount RR in this case is made the amount at
the mean temperature in the intermediate temperature region. In
this regard, this NO.sub.X reduction amount RR increases the higher
the catalyst temperature TC becomes. Therefore, as shown in FIG.
23, when there is a part in the exhaust purification catalyst 13
where the temperature TC is high, the NO.sub.X reduction amount RR
increases. Therefore, in this embodiment, the NO.sub.X amount NOX
which is stored per unit time is calculated based on the following
formula, and when in an operating state where there is a part in
the exhaust purification catalyst 13 where the temperature is high
as shown in FIG. 23, the value of the increase coefficient ZK,
which is usually made 1.0, is increased.
NOX=(NOXA-RRZK)KR
When, in this way, in this embodiment, a temperature difference
arises in the exhaust purification catalyst 13 and there is a
temperature region which is higher than the temperature TC of the
exhaust purification catalyst 13 which is detected in the exhaust
purification catalyst 13, the NO.sub.X reduction amount RR
increases.
[0088] Note that, as another embodiment, it is also possible to
arrange an oxidation catalyst for reforming the hydrocarbons in the
engine exhaust passage upstream in the exhaust purification
catalyst 13.
REFERENCE SIGNS LIST
[0089] 4 intake manifold [0090] 5 exhaust manifold [0091] 7 exhaust
turbocharger [0092] 12 exhaust pipe [0093] 13 exhaust purification
catalyst [0094] 14 particulate filter [0095] 15 hydrocarbon feed
valve
* * * * *